As a known shape of an electric double layer capacitor, there may, for example, be a coin type wherein is an element having a separator sandwiched between a pair of polarized electrodes composed mainly of activated carbon formed on current collectors, is accommodated together with an electrolytic solution in a metal casing, which is then sealed by a metal cover via a gasket, or a cylindrical type wherein an element having a pair of polarized sheet electrodes wound with a separator interposed therebetween, is accommodated together with an electrolytic solution in a metal casing, which is then sealed so that the electrolytic solution will not evaporate from an opening of the casing.
Further, as one for a large current and large capacitance application, a stacking type electric double layer capacitor has also been known wherein an element having many polarized sheet electrodes stacked via a separator disposed therebetween, is incorporated (Patent Documents 1 to 3). In such a stacking type electric double layer capacitor, rectangular polarized sheet electrodes are used as positive electrodes and negative electrodes, and they are alternately stacked with a separator interposed therebetween, to form an element, which is then accommodated in a casing in such a state that positive electrode lead members and negative electrode lead members are connected by caulking to the ends of the positive and negative electrodes, respectively, then the element is impregnated with an electrolytic solution, and the casing is sealed with a cover.
As an electrolytic solution for an electric double layer capacitor, not only an aqueous electrolytic solution containing e.g. a mineral acid such as sulfuric acid, an alkali metal salt or an alkali, but also various non-aqueous electrolytic solutions (organic electrolytic solutions) have been used. The non-aqueous electrolytic solution has a withstanding voltage of from 2.5 to 3.3 V, while the aqueous electrolytic solution has a withstanding voltage of 0.8 V. The electrostatic energy of a capacitor is proportional to the square of the withstanding voltage. Accordingly, from the viewpoint of the electrostatic energy, the non-aqueous electrolytic solution is more advantageous.
A solvent to be used for the non-aqueous electrolytic solution is necessary to highly dissolve an electrolyte, and therefore a solvent having high dielectric constant is preferred. Specifically, propylene carbonate, γ-butyrolactone, acetonitrile, dimethylformamide (Patent Document 4), sulfolane or its derivative (Patent Document 5) or fluorobenzene (Patent Document 6) has been known.
However, such a solvent having a high dielectric constant usually has a high viscosity, and therefore there was a problem that it was impossible to obtain high electric conductivity.
On the other hand, a chain ether, a chain ester, a chain carbonate and the like, which are low viscosity solvents, have a low dielectric constant, whereby substantially no electrolyte will be dissolved therein, and therefore there was a problem that no high electric conductivity was obtained.
Propylene carbonate which is commonly used as a solvent for a non-aqueous electrolytic solution, is a solvent having a high dielectric constant and a relatively not so high viscosity at room temperature, and is thereby preferably used.
However, a non-aqueous electrolytic solution in which propylene carbonate is used as a solvent and a salt comprising as a cation (C2H5)4N+, (C2H5)4P+, (C2H5)3(CH3)N+ or the like having a high electric conductivity is used as an electrolyte, has a relatively good electric conductivity at around room temperature, but the electric conductivity remarkably decreases at low temperature, and therefore there was a problem that an electric double layer capacitor using such a non-aqueous electrolytic solution was poor in low temperature characteristics.
On the other hand, Patent Document 7 discloses a non-aqueous electrolytic solution for an electric double layer capacitor, characterized by containing, as an electrolyte, spiro quaternary ammonium tetrafluoroborate in a mixed solvent containing dimethyl carbonate and propylene carbonate, for the purposes of low viscosity and excellent low temperature characteristics.
Further, in Patent Document 8, the present inventors have proposed an electrolytic solution comprising as an electrolyte, a pyrrolidinium cation represented by RaRbRcRdN+ (wherein each of Ra and Rb which are independent of each other, is a C1-4 alkyl group, and Rc and Rd together form a tetramethylene group to provide a cyclic pyrrolidinium cation), and as solvents, dimethyl carbonate, sulfolane and fluorobenzene.
In such an electrolytic solution, the above electrolyte having the pyrrolidinium cation is highly soluble in dimethyl carbonate as a solvent having a low dielectric constant and a low viscosity, sulfolane is added thereto as a solvent to improve the uniformity of the solution in a region of low electrolyte concentration, and further fluorobenzene which has a low melting point of −42° C. and which is a low viscosity solvent is added thereto as a solvent, whereby it is possible to realize an electric double layer capacitor having a low resistance and running also in low temperature region.
Patent Document 1: JP-A-4-154106 (Claims, FIG. 1)
Patent Document 2: JP-A-3-203311 (Claims, FIG. 3)
Patent Document 3: JP-A-4-286108 (claim 1)
Patent Document 4: JP-A-49-068254 (Claims)
Patent Document 5: JP-A-62-237715 (Claims)
Patent Document 6: JP-A-2004-6803 (Claims)
Patent Document 7: JP-2006-49447 (Claims)
Patent Document 8: WO05/008700 (Claims)